Phase delay plates can now be manufactured for the visible spectral range approximately up to a diameter of 300 mm with excellent retardation tolerance of .+-.5% for .lambda./4 and with an excellent optical throughput. For this purpose, stretched polyvinyl alcohol foil is cemented between optical plates. Unfortunately, these foils are not applicable for projection microlithography with deep ultraviolet light, for example of the wavelength 193 nm, because neither cement nor foil are stable under the radiation load.
The crystalline materials, which are double refracting and can be used, are SiO.sub.2 and MgF.sub.2. If .lambda./4 plates having a diameter of approximately 200 mm are needed, then it is very improbable to obtain suitable material for SiO.sub.2. A retardation plate having a simple thickness is ideal. Here, the thickness is computed in accordance with the formula: EQU d=(k.multidot..lambda.):.DELTA.n.
The resulting plate thicknesses are almost the same for SiO.sub.2 and MgF.sub.2. For a quartz crystal, the difference of the index of refraction .DELTA.n=0.0135 at 193 nm. In this way, a plate thickness of d=3.57 .mu.m results. This is very low. Although quartz has an excellent wringing property (property which enables two surfaces to adhere when in contact due to molecular cohesion), at least a triple thickness should be sought in order to achieve a certain mechanical stability at 10 .mu.m. This thickness is, however, not acceptable because of optical disturbance of the objective.
The Carl Zeiss Company of Oberkochen, Germany, has produced a .lambda./4 plate of quartz having a diameter of 150 mm and a thickness of approximately 16 .mu.m for a wavelength of 633 nm.
The actual problem resides in the availability of SiO.sub.2 in the required size. Synthetic crystals achieve the size as plates; however, the crystal axis is perpendicular to the plane of the plate and not in the plate plane as required. The growth speeds in crystal growing are clearly different and are the slowest especially in the direction of the crystal axis. Synthetic pieces having a dimension in the direction of the crystal axis of greater than 100 mm are not known because of the reasons associated therewith.
Natural quartz crystals can, in individual cases, achieve the required size. In this way, no reliable production can be achieved because all known natural deposits are exhausted.
The alternative to quartz is synthetically produced MgF.sub.2. However, here too problems are present in growing pieces of this size because MgF.sub.2 has different thermal coefficients of expansion for the different directions in the crystal. Large crystals must be carefully thermally grown so that they do not burst. In this context, it is additionally difficult that MgF.sub.2 cannot be wrung. Accordingly, cement must be used during processing. Later, a plate having a thickness of only a few micrometers must be held in optical contact between two quartz glass plates with an immersion medium.
The simple plate thickness for MgF.sub.2 is 3.54 .mu.m or .DELTA.n=0.0136 of MgF.sub.2 at 193 nm.
The optical phase retardation can also be achieved by pressing or stretching a material which is at first isotropic. In this way, stress-induced birefringence is generated. Only materials transparent for deep ultraviolet light (especially for 193 nm) available in the required diameter, are suitable. Quartz glass can also be utilized because the radiation load is reduced as a consequence of the large diameter.
The question as to whether compression or tension is to be used is answered in favor of tension. With compression, the plates can bend which makes them fully unusable in a projection objective. With tension, the planarity and therefore the optical passthrough is improved.